Digital printing apparatus using liquid development dispersion—also known as liquid toner—are known from US patent application publication no. 2011/0249990. The known digital printing apparatus comprises a feed member, a development member, development member cleaning means, and an image carrying member, these member being preferably rollers; the feed member being arranged to transfer a quantity of liquid toner from a toner container onto the toner member; and the development member being arranged to transfer a portion of the quantity of liquid toner onto the image carrying member in accordance with a charge pattern sustained on a surface of said image carrying member. A liquid toner residue, also referred to as an excess liquid toner dispersion, remains on (the surface of) the development member after the image wise transfer of the liquid toner from the development member to a further member, particularly the imaging member.
In digital printing systems of this kind, it is necessary to remove the liquid toner residue that remains on the surface of the development member after contact with the imaging roller. Any liquid toner residue that remains on the surface of the imaging member after contact with a transfer roller of after contact with a substrate needs to be removed as well. More generally, it may be desirable to remove a residue remaining on any member of the apparatus. It is observed that these highly concentrated and therefore highly viscous compacted toners are not easily decompacted and removed from such members. Thus, the removal of such a residue can be quite challenging and therefore it is better to try to prevent such situation from happening.
Particularly, toner particles in the liquid toner dispersion tend to form lumps in the dispersion resulting in a liquid with a non-uniform distribution of toner particles. This is called caking and often results in an increase of the viscosity of the liquid dispersion. This viscosity increase is significant and could be a tenfold increase or even more.
Liquid toner dispersion that shows caking cannot be used for printing as such and needs to be treated first in order to re-obtain a homogeneously dispersed liquid toner which has similar conductivity and viscosity properties as the starting liquid toner dispersion. The treatment process therefore needs to be monitored and arranged such that the properties of the liquid toner dispersion in the toner container remain appropriate and should bring the toner dispersion in such a condition that the reoccurrence of caking is maximally suppressed. One example of a process thereof is known from US2011/0103840A1 that is included herein by reference.
It is thought that caking is the result of toner particles that come so close into each other's neighbourhood on the developing member, so that they start to feel each other's presence and start interacting with each other. Caking can also be the result of injecting charge and applying high shearing forces which are typically present when a thin layer of liquid toner dispersion passes through a very narrow gap between two (rotating) members of the printing apparatus or huge (microsized) mechanical interaction like a cleaning blade scraping on a rotating surface.
The removal of the liquid toner residue starts then to be problematic. As a result, liquid toner residue could remain on the development member or a scraper blade, which constitutes a contamination and may lead to a non-uniform distribution of fresh toner dispersion resulting in a ghost image, density fluctuations and or image quality that is not perfect, in other words incorrect. The cleaning performance of the development member also is then negatively influenced and failing may occur. Specific examples of issues are density instability and incorrect reproduction of fine lines or background. Removal of the toner residue by a removal device may reduce the issue, but is known to solve the issue incompletely. It is therefore a major problem to solve the caking issue, without giving rise to a reduced printing quality.